1. Field of the Invention
The present invention relates to a sensor device represented by an infrared sensor and, more particularly, to its signal detection circuit.
2. Description of the Related Art
An image sensor formed by two-dimensionally arraying infrared sensors can be used even at night. However, its sensitivity or dynamic range is poorer than that of an image sensor using visible light. For example, a monitor camera requires an NETD (Noise Equivalent Temperature Difference) of about 1 K and a dynamic range of about 30 K. It is therefore necessary to develop an infrared sensor having a higher sensitivity and wider dynamic range.
An infrared sensor array is formed by arraying a plurality of infrared sensors on a silicon substrate. Each infrared sensor is constituted by an infrared absorption section and a diode (normally formed by series connection of a plurality of diodes) serving as a thermoelectric conversion element which converts heat generated in the infrared absorption section into an electrical signal. In a bolometer non-cooled infrared sensor device, each sensor is held in a floating state to effectively transmit generated heat to the diode. Such a floating support structure is made by micromachinining technology.
Infrared rays radiated from an object are condensed by an optical lens arranged in front of the sensors and increase the temperature of the diode of each sensor. As the optical lens, a film made of a material having a high infrared transmission efficiency, for example, a Ge film is used. Assume, for example, that, a Ge lens having a transmittance of 90% and an F-number of 1.0 at a wavelength of 8 to 12 μm is used. When the surface temperature of an object changes by 1 K, the diode temperature increases by about 1×10−3K. The diode is driven such that a constant current is supplied from a constant current source. The current density J in the diode is given as a function of the temperature T, i.e., J=Js(eqV/kT−1), Js=T3+γ/2×exp(−Eg/kT), where k is the Boltzmann constant, Eg is the bandgap of silicon, and γ is a predetermined constant.
When the diode temperature increases, the voltage drop Vf of the diode decreases. Assume that when the temperature of eight diodes connected in series increases by 1K, the voltage drop Vf is about 20 mV/K. When the surface temperature of an object increases by, for example, 30 K, a potential difference of about 0.618 mV is generated as a voltage drop. When this potential difference is detected and output as a sensor output, an infrared image is detected.
In a conventional bolometer non-cooled infrared sensor, since the sensor output characteristic with respect to the change in diode temperature is not linear, it is difficult to simultaneously realize high sensitivity and a wide dynamic range.